Image data obtained by inputting an image by an image input device such as a digital still camera, a movie camera or a scanner has a bit depth of, e.g. 8 bits, for example, for each color of B (blue), G (green) and R (red). Thus, an image with a tone characteristic close to that of conventional silver-salt pictures can be obtained. In this specification, such an image is called a picture quality image.
Image data are excellent in processability since being numerical data. Taking advantage of this characteristic, various processing is applied to image data to obtain images expressed in more various manners. As an example is known a method for obtaining a painterly image by intentionally reducing the tone number (i.e. number of tones or gradations) of a picture quality image. In reducing the tone number of the picture quality image, a posterization processing is performed. A tone curve is applied in changing the tone characteristic of an image. The tone curve determines a tone conversion characteristic in determining each tone value (output value) of an output image in correspondence with each tone value (input value) of an input image. By setting the shape of this tone curve not to be a continuous straight line or curve, but to be a discrete step function, a posterized output image can be obtained.
The posterization processing is performed to divide a range of input values from a minimum value to a maximum value into a finite number of zones and determine an output value corresponding to each divided zone as a tone conversion characteristic. The posterization processing derives pixel values of an output image in correspondence with those of an input image based on the tone conversion characteristic. As a result of the posterization processing, pixel values falling within a certain zone of an input image are consolidated into one pixel value representing that zone. As a result, the tone of a generated image becomes discrete (stepwise) and an image like a painting can be obtained.
JP01-314389A1 discloses a technology for color converting R, G and B data into hue, lightness and saturation data, reducing the tone numbers of the hue, lightness and saturation data and inversely converting these data into R, G and B data again in obtaining an illustrated image by performing a posterization processing.